Axeing the HS2 link between Birmingham and Manchester would be "very stupid", according to an industry expert.
Richard Bowker, the former Strategic Rail Authority head, also said not taking the HS2 line into central London would be "utter madness".
Rising costs have led to speculation around its future and the government has not guaranteed the line will run to the North West.
A Home Office minister said it was looking at how costs can be controlled.
The high speed rail project is intended to link London, the Midlands and the north of England. The first part, between west London and Birmingham, is already under construction.
But the scheme as a whole has already faced delays, cost increases and cuts - including the planned eastern leg between Birmingham and Leeds which was axed in late 2021.
Mr Bowker told the BBC Radio 4's Today programme that if the Birmingham to Manchester section of HS2 was dropped, "we really have done a very, very stupid thing".
"HS2 to Manchester is fully integrated with something called Northern Powerhouse Rail. You can't do Northern Powerhouse Rail without that Manchester section."
The Northern Powerhouse Rail project would include a mix of new and upgraded lines to speed up links between Liverpool, Manchester and Leeds. It plans to use a section of the HS2 line from Manchester Airport to Manchester Piccadilly, as well as the planned upgrades to Manchester Piccadilly station.
The Conservative Party is holding its annual conference in Manchester from Sunday, 1 October.
There had been speculation that an announcement concerning HS2 could be made as soon as this week although the government may hold off until the Autumn Statement on 22 November.
As well as the link from Birmingham to Manchester being in doubt, there are also questions over whether the line will end at Old Oak Common in west London, rather than carry on to Euston station in central London.
Mr Bowker said that to not complete the railway to London would be "utter madness".
"Old Oak Common was never designed as a terminus station, it's a through-station that will cost us billions to eventually fix that problem if we don't go to London," he said.
The Times reported on Tuesday that Prime Minister Rishi Sunak has been "alarmed" by the escalating cost of the HS2 project, with suggestions that it could eventually exceed 拢100bn.
The report also said that if the northern leg of HS2 is cancelled, the prime minister was considering reallocating money to other regional transport projects such as Northern Powerhouse Rail.
Esther McVey, the Conservative MP for Tatton in Cheshire, told the Today programme that cancelling the Manchester leg of HS2 was "the right thing to do".
"Things have significantly changed since lockdown, people would now sooner jump on a Zoom to save time and money," she said.
HS2 route map
She said the money would be better spent on connecting northern cities and and improving local transport "because HS2 is sucking the money and the life out of our local transport".
But the new co-owner of Birmingham City Football Club, Tom Wagner, said that abandoning the link between Birmingham and Manchester "could result in a loss of investor trust".
In a letter to Mr Sunak, Mr Wagner said that the decision to take over the Championship's longest-serving club back in July was partly influenced by "the exciting plans to improve connectivity with the rest of the UK through projects including HS2".
He said: "Any deviation could result in a loss of investor trust and this would have a considerable negative impact on the UK. The ambitious HS2 project falls into this category."
Meanwhile, Birmingham Airport chief executive Nick Barton, has also written to the prime minister, saying such a move would be "a mistake" which would "short change every generation to follow".
Speaking to Sky News, Home Office Minister Chris Philp said: "No decisions have been taken about the remaining stages of HS2 but I do know the chancellor and the prime minister are looking at how the cost can be controlled.
"The commitment to the Midlands, the North, the levelling up agenda is absolutely undimmed."
The last official estimate on HS2 costs was 拢71bn, excluding the cancelled eastern route. But this was in 2019 prices, so does not take into account the rise in costs for materials and wages since then.
